Image decoding device and image decoding method

ABSTRACT

After completion of a prediction process for a single line in an image of size S, the DC components and AC components, held in a lower stage part of a predicted luminance value storage are copied into a line part. This copying is done to use the DC components and AC components that have been copied into line part as reference values for the prediction process of the target macro blocks of the next single line. Performing the prediction process while repeating such copying eliminates the need to secure the area for storing of the DC components and AC components of the entire image size in predicted luminance value storage for the prediction process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns an image decoding device and method fordecoding, in target macro block units, coded image signals that arecoded by DCT, quantization, and variable-length coding.

2. Description of the Related Art

In the field of image coding technology, a standard called MPEG4 isspecified and standardized in the specifications titled, “ISO/IEC14496-2Information Technology—Generic Coding of Audio-Visual Object.”

However, this document does not describe methods of implementing thedecoding of image signals coded in accordance with MPEG4.

Also, though the book titled “MPEG-4 No Subete” (Japanese term meaning“All About MPEG4”; Miki et al, 1998, published by Kogyo ChosakaiPublishing Co., Ltd.) provides a description of the principles of adecoding method for this type of coded signal, it does not providespecific methods of implementation. Besides the above, substantialliterature concerning specific implementation methods could not befound.

Thus, a specific method of implementation is now considered. First asdescribed in the abovementioned book, an entire image size is comprisedby arranging areas of 16×16 pixels, called macro blocks, vertically andhorizontally. Each macro block is associated with a total of six typesof data, that is, the luminance values (Y1, Y2, Y3, and Y4) concerningthe four blocks (each comprised of 8×8 pixels) at the upper left, upperright, lower left, and lower right and two color difference values (Cband Cr). These data are processed in the order, Y1, Y2, Y3, Y4, Cb, andCr.

With regard to the prediction process for decoding coded image signals,the abovementioned specifications do not indicate limits concerning thememory area to be used for the prediction process. Therefore, thismemory area may be set freely.

The method of securing a memory area corresponding to the image size asthe memory area to be used for the prediction process may thus beconsidered.

OBJECTS AND SUMMARY OF THE INVENTION

In actuality, an image processor that realizes a prediction process islimited by its storage capacity and cannot carry an unlimited amount ofmemory.

Increasing the memory carried increases the consumption power of theimage processor and increases of the chip size. This decreases theduration of continuous use of the equipment loaded with the processorand increases the production cost of the processor.

It is therefore an object of this invention to provide an image decodingdevice and method by which the memory area to be used for the predictionprocess in the decoding of coded image signals is reduced.

A first mode of this invention provides an image decoding device, whichdecodes in target macro block units, coded image signals that are codedby DCT, quantization, and variable-length coding, an image decodingdevice consisting of a target macro block setting means, which sets thetarget macro block to be decoded currently from among the entire imagesize, a target macro block extraction means, which performs a variablelength decoding process and a zigzag scan on the coded image signal andextracts the data of the target macro block, a prediction means, whichperforms a DC/AC prediction process on the extracted target macro blockdata, an inverse quantization means, which performs inverse quantizationon the target macro block data that have been subject to the predictionprocess, and an inverse DCT means, which performs inverse DCT on thetarget macro block data that have been subject to inverse quantizationand outputs the image of this target macro block, and characterized inthat the prediction means is equipped with a reference value storagemeans, which holds reference values that are necessary for theprediction process performed on the target macro block, a predictioncomputation means, which performs prediction computation based on thereference values of the reference value storage means, a predicted valuestorage means, which holds the prediction computation results, and aprediction control means, which controls the reference value storagemeans, prediction computation means, and predicted value storage means,the total storage volume of the reference value storage means andpredicted value storage means is smaller than the storage volume for thepredicted values for the entire image size, and the prediction controlmeans copies the data, necessary for the prediction computation of thenext target macro block, from the predicted value storage means to thereference value storage means.

With the above arrangement, the data necessary for the next predictionprocess can be copied from the predicted value storage means to the areain the reference value storage means in which the reference values thathave become unnecessary for the prediction process are held. The nextprediction process is then performed using the data that have beencopied into the reference value storage means.

Performing the prediction process while repeating such a copyingprocedure reduces the volume of the reference value storage required inthe reference value storage means and eliminates the need to secure anarea for storing the predicted values for the entire image size in thepredicted value storage means for the prediction process. As a result,the memory area to be used for the prediction process in the decoding ofcoded image signals is reduced.

With an image decoding device of a second mode of this invention, thereference value storage means is consists of a line part, which holdsthe DC components and AC components for a single line in the image size,a corner part, which holds one DC component, and a left part, whichholds the DC components and AC components of the block locatedimmediately prior and adjacently to the left side of the target macroblock.

With this arrangement, the DC components and AC components necessary forthe prediction process of target macro blocks of a single line can becopied into the line part.

The DC components and AC components that have been copied into the linepart can thus be used as reference values for the prediction process ofthe target macro blocks of the next single line.

Performing the prediction process by repeating such a copying processthus eliminates the need to hold the DC components and AC componentsnecessary for the prediction process of the target macro blocks of thenext single line in the predicted luminance value storage means.

Furthermore, with this arrangement, the data necessary for theprediction process of the current target macro block, that is, the DCcomponents and AC components of the block, located immediately prior andadjacently to the left side, can be copied into the left part.

The DC components and AC components, which have been copied into theleft part, can thus be used in the prediction of the current targetmacro block.

Performing the prediction process by repeating such a copying processeliminates the need to hold the DC components and AC components of theblock, located immediately prior and adjacently to the left side of thecurrent target macro block, for the prediction process of the currenttarget macro block in the predicted value storage means.

As a result of the above, it is sufficient to secure just an area forholding at least the DC components and AC components of one target macroblock in the predicted luminance value storage means.

The memory area used for the prediction process in the decoding of codedimage signals is thus further reduced.

With an image decoding device of a fourth mode of this invention, theleft part is allocated to two sets of DC components and AC componentsfor the four luminance components of the target macro block.

This arrangement provides more adequately for the case where the targetmacro block consists of four luminance components in accordance withstandardized specifications. In particular, this is favorable for theprediction process in decoding image signals coded in accordance withMPEG-4 specifications.

With an image decoding device of a fifth mode of this invention, thepredicted value storage means consists of areas that can hold the DCcomponents and AC components of the respective positions in a process inwhich the target macro block is moved by one line in the image size.

By this arrangement, the copying by the prediction control means can beperformed in a batch of one line at a time to simplify the process.

With an image decoding device of a sixth mode of this invention, thepredicted value storage means consists of an area that can hold the DCcomponents and AC components of one target macro block.

By this arrangement, the copying by the prediction control means can beused in the most rational manner and the memory area used for theprediction process is reduced significantly.

A seventh mode of this invention provides, in an image decoding methodfor decoding, in target macro block units, coded image signals that arecoded by DCT, quantization, and variable-length coding, an imagedecoding method comprised of a first step of setting the target macroblock to be decoded currently, a second step of performing a variablelength decoding process and a zigzag scan on the coded signal andextracting the data of the target macro block, a third step ofperforming a DC/AC prediction process on the extracted target macroblock data, a fourth step of performing inverse quantization on thetarget macro block data that have been subject to the predictionprocess, and a fifth step of performing inverse DCT on the target macroblock data that have been subject to inverse quantization and outputtingthe image of this target macro block, and in the third step, a storagearea, which is smaller in storage volume than the storage volume for thepredicted values for the entire image size, is used and the predictionprocess is performed while copying the data, necessary for theprediction computation of the next target macro block, into this storagearea.

Performing the prediction process while copying into a storage area thatis smaller in storage volume than the storage volume for the predictedvalues for the entire image size in the above manner eliminates the needto secure an area for storing the predicted values of the entire imagesize for the prediction process. As a result, the memory area used forthe prediction process in the decoding of coded image signals isreduced.

The above, and other objects, features and advantages of the presentinvention will become apparent from the following description read inconjunction with the accompanying drawings, in which like referencenumerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an image decoding device according to anembodiment of this invention.

FIG. 2 is a block diagram of a prediction means of an embodiment of thisinvention.

FIG. 3 is a diagram to which reference will be made in explaining the DCprediction of an embodiment of this invention.

FIG. 4 is a diagram to which reference will be made in the AC predictionof an embodiment of this invention.

FIG. 5 is a flowchart, which shows the prediction computation of anembodiment of this invention.

FIG. 6 is a flowchart, which shows the image decoding method of anembodiment of this invention.

FIG. 7 is a flowchart, which shows the prediction process of Embodiment1 of this invention.

FIGS. 8 and 9 are diagrams to which reference will be made in theluminance prediction process of Embodiment 1 of this invention.

FIG. 10 is a diagram to which reference will be made in the chrominanceprediction process of Embodiment 1 of this invention.

FIG. 11 is a diagram to which reference will be made in a specific DC/ACprediction process of Embodiment 1 of this invention.

FIG. 12 is a diagram to which reference will be made in the copying bythe prediction control means of Embodiment 1 of this invention.

FIG. 13 is a flowchart, which shows the prediction process of Embodiment2 of this invention.

FIGS. 14 through 18 are diagrams to which reference will be made in theluminance prediction process of Embodiment 2 of this invention.

FIG. 19 is a diagram to which reference will be made in the chrominanceprediction process of Embodiment 2 of this invention.

FIG. 20 is a diagram to which reference will be made in the copying bythe prediction control means of Embodiment 2 of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Embodiment 1)

Referring to FIG. 1, an image decoding device of an embodiment of theinvention, shown generally at 1, decodes and outputs the resulting datato an image memory 2. The data entering the image memory 2 is in targetmacro block units that are coded image signals coded by DCT (discretecosine transform), quantization, and variable-length coding.

Image decoding device 1 is equipped with a target macro block settingmeans 3, a target macro block extraction means 4, a prediction means 6,an inverse quantization means 7, and an inverse DCT means 8.

Target macro block setting means 3 selects the target macro block to bedecoded from among the entire image size, for example, by designatingposition information or a block number, etc. Target macro blockextraction means 4 performs a variable length decoding process and azigzag scan on the coded image signal and extracts the data of theselected target macro block.

Prediction means 6 performs a DC/AC prediction process on the extractedtarget macro block data. Inverse quantization means 7 performs inversequantization on the target macro block data that have been subject tothe prediction process. Inverse DCT means 8 performs an inverse DCTprocess on the target macro block data that have been subject to inversequantization and outputs the image of this target macro block to imagememory 2.

When the inverse DCT is completed, inverse DCT means 8 notifies thisfact to target macro block setting means 3. Upon receiving thisnotification, target macro block setting means 3 steps theidentification of the target macro block to the next target macro block.

Referring now to FIG. 2, prediction means 6, shown in FIG. 1, includes aprediction control means 10, a reference luminance value storage means11, a reference chrominance value storage means 12, a predictedluminance value storage means 13, a predicted chrominance storage means14, and a prediction computation means 15. The storage means 11, 12, 13,and 14 may be arranged as separate storage media or they may be arrangedas separate areas in a single memory medium (for example, a memory).

Reference luminance value storage means 11 holds reference valuesnecessary for the prediction process concerning the luminance of thetarget macro block. Reference chrominance value storage means 12 holdsreference values necessary for the prediction process concerning thechrominance of the target macro block.

Prediction computation means 15 performs prediction computation based onthe reference values of reference luminance value storage means 11 andreference chrominance value storage means 12. Predicted luminance valuestorage means 13 holds the results of prediction computation based onthe reference values of reference luminance value storage means 11.Predicted chrominance value storage means 14 holds the results ofprediction computation based on the reference values of referencechrominance value storage means 12.

Prediction control means 10 controls reference luminance value storagemeans 11, reference chrominance value storage means 12, predictedluminance value storage means 13, predicted chrominance value storagemeans 14, and prediction computation means 15.

Though a more detailed description is provided below, the total of thereference value storage volumes of reference luminance value storagemeans 11 and reference chrominance value storage means 12 and thepredicted value storage volumes of predicted luminance value storagemeans 13 and predicted chrominance value storage means 14 is greatlyreduced from the storage volume that would be required for the predictedvalues for the entire image size.

Prediction control means 10 copies data, which are necessary for theprediction computation concerning the target macro block, from predictedluminance value storage means 13 and predicted chrominance value storagemeans 14 into reference luminance value storage means 11 and referencechrominance value storage means 12, respectively.

The principles of the DC/AC prediction (principles of predictioncomputation) carried out in prediction means 6 are now described withreference to FIGS. 3 through 5.

When a prediction process on a target block X is to be carried out, thethree blocks at the upper side C, left side A, and diagonally above Bthe block on which a prediction is to be carried out are referenced.When target block X is subjected to the prediction process, the threeblocks of block A, block B, and block C (each comprised of 8×8 pixels)are referenced.

To be more specific, in step 1, reference values Fa[0][0], Fb[0][0], andFc[0][0] are acquired as shown in FIG. 3 and FIG. 5. Here, the suffix ain Fa[0][0] indicates a block in FIGS. 3 and 4, and in this case,indicates the block A.

The [0] at the left side of Fa[0][0] indicates a column in a block inFIGS. 3 and 4, and in this case, indicates the 0 column. The [0] at theright side of Fa[0][0] indicates a row in a block in FIGS. 3 and 4, andin this case, indicates the 0 row. The same holds true for referencevalues Fb[0][0] and Fc[0][0].

In step 2, the value of dif is determined asdif=|Fa[0][0]−Fb[0][0]|−|Fb[0][0]−Fc[0][0]|. ∥ signifies the absolutevalue.

If in step 3, dif <0, step 4 is entered and the reference block is setto block C. Thus, in this case, the prediction process is carried outbased on reference block C. This determines the direction of prediction.

In step 4, the values of Val and Qref are set as Val=Fc[0][0] andQref=Qpc.

Val is the prediction reference value, Qref is the quantization factorreference value, and Qpc is the quantization factor of block C.

If in step 3, dif is not <0, step 5 is entered and the reference blockis set to block A. Thus, in this case, the prediction process is carriedout based on reference block A.

In step 5, the values of Val and Qref are set as Val=Fa[0][0] andQref=Qpa. Here, Qpa is the quantization factor of block A.

In step 6, the predicted value (DC component) of target block X isdetermined. To be more specific, predicted valueFx[0][0]=PFx[0][0]+(Val+Qref)//Qpx. Here, PFx indicates data of targetblock X, which has been subject to a variable length decoding process,and are input from target macro block extraction means 4 (see FIG. 1).The operator // indicates “rounding in the direction of 0”. Qpx is thequantization factor of block X.

Here, the meanings of the suffix x and [0][0] in Fx[0][0], PFx[0][0] arethe same as the meanings given above.

If the reference block was set to block C in step 4, step 8 is enteredat step 7. Then in step 8, the predicted values (AC components) oftarget block X are determined as shown in FIGS. 4 and 5. To be morespecific, predicted values Fx[j][0] are determined asFx[j][0]=PFx[j][0]+(Fa[j][0]×Qref)//Qpx. Here, j=1, 2, . . . , 7.

If the reference block was set to block A in step 5, step 9 is enteredat step 7. Then in step 9, the predicted values (AC components) oftarget block X are determined. To be more specific, predicted valuesFx[0][i] are determined as Fx[j][0]=PFx[0][i]+(Fa[0][i]×Qref)//Qpx.Here, i=1, 2, . . . , 7.

In step 10, the predicted values of target block X are returned toprediction control means 10. The processes from step 1 through step 10are carried out by the prediction computation means 15 shown in FIG. 2.

Thus as is described above, for the prediction process concerning targetblock X, the block C at the upper side, the block A at the left side,and the block B diagonally above are used.

The overall flow of the image decoding method of image decoding device 1is now described.

Referring now to the flowchart in FIG. 6, the image decoding methodbegins in step 11 where prediction control means 10 performs initialsetting of the reference values held in reference luminance valuestorage means 11 and reference chrominance value storage means 12. To bemore specific, the DC components are set equal to 1024 and the ACcomponents are set equal to 0.

In step 12, target macro block setting means 3 sets the target macroblock to be decoded first from among the entire image size (initialsetting).

In step 13, target macro block extraction means 4 performs a variablelength decoding process (VLD: variable length decoding) and a zigzagscan on the coded image signal and extracts the data of the target macroblock. In step 14, prediction means 6 performs a DC/AC predictionprocess on the extracted target macro block data.

In step 15, inverse quantization means 7 performs inverse quantization(IQ: inverse quantization) on the target macro block data that have beensubject to the prediction process. In step 16, inverse DCT means 8performs inverse DCT on the target macro block data that have beensubject to inverse quantization. In step 17, inverse DCT means 8 outputsthe image of the target macro block that is obtained in step 16.

If in step 18, the processes from step 13 through 17 have not beencompleted for all macro blocks, step 19 is entered. In step 19, targetmacro block setting means 3 steps the target macro block to the nextmacro block. Target macro block extraction means 4 then extracts thedata of the renewed target macro block from the coded image signal.

The processes of steps 13 through 17 are then carried out on theextracted target macro block data. If in step 18, the processes fromstep 13 through 17 have been completed for all macro blocks, thedecoding of the coded image signal by this device is ended.

The prediction process is now described in detail with reference toFIGS. 7 through 10.

Referring specifically to FIG. 8, reference luminance value storagemeans 11 has a line part 23, which holds the DC components and ACcomponents of a single line in image size S (which is determined by thenumber of vertical and horizontal pixels of one frame), a corner part21, which holds a single DC component, and a left part 22, which holdsDC components and AC components. The DC components and AC componentsheld by reference luminance value storage means 11 are reference valuesto be used in the process of carrying out the prediction processconcerning luminance.

For the four luminance components of each target macro block, line part23 has an area allocated for two sets of DC components and ACcomponents. To be more specific, in the present example, since there arefive target macro blocks in a single line in image size S, line part 23is comprised of memory areas that hold ten DC components and ten ACcomponents.

The storage area that holds one DC component in one of theabovementioned sets is a storage area for one pixel, which is used tostore the reference value Fc[0][0] indicated in FIG. 3 or 4, and thestorage area that holds one AC component in one of the abovementionedsets is a storage area for seven pixels, which is used to store thereference values Fc[j][0] (j=1, 2, . . . , 7) indicated in FIG. 4.

Left part 22 is allocated for two sets of DC components and ACcomponents for the four luminance components of the target macro block.To be more specific, left part 22 consists of memory areas that hold twoDC components and two AC components.

The storage area that holds one DC component in one of theabovementioned sets is a storage area for one pixel. This one-pixelstorage area is used to store the reference value Fa[0][0] indicated inFIG. 3 or 4. The storage area that holds one AC component in one of theabovementioned sets is a storage area for seven pixels, which is used tostore the reference values Fa[0][i] (i=1, 2, . . . , 7) indicated inFIG. 4.

Predicted luminance value storage means 13 consists of memory areas thathold the DC components and AC components of the luminance of the targetmacro blocks of a single line in image size S. Put another way,predicted luminance value storage means 13 consists of areas for holdingthe DC components and the AC components of the respective positions in aprocess in which the target macro block is moved by one line. As ismentioned above, the DC components and AC components held in predictedluminance value storage means 13 are predicted values obtained as aresult of the prediction process based on luminance.

Referring now to FIG. 10, the details of the reference chrominance valuestorage means 12 and predicted chrominance value storage means 14, whichare shown in FIG. 2 and are necessary for carrying out the predictionprocess, are now described.

Reference chrominance value storage means 12 has a line part 43, whichholds the DC components and AC components of a single line in image sizeS, a corner part 41, which holds a single DC component, and a left part42, which holds DC components and AC components. The DC components andAC components held by reference chrominance value storage means 12 arereference values to be used in the process of carrying out theprediction process concerning chrominance.

For each target macro block, line part 43 has an area allocated for oneset of DC components and AC components. To be more specific, in thepresent example, since there are five target macro blocks in a singleline in image size S, line part 43 consists of memory areas that holdfive DC components and five AC components.

Left part 42 is allocated for one set of DC components and AC componentsfor the target macro block. To be more specific, left part 42 consistsof memory areas that hold one DC component and one AC component.

Predicted chrominance value storage means 14 of FIG. 2 consists ofmemory areas that hold the DC components and AC components of thechrominance of the target macro blocks of a single line in image size S.Put in another way, predicted chrominance value storage means 14consists of areas that can hold the DC components and AC components ofthe respective positions in a process in which the target macro block ismoved by one line. The DC components and AC components held in predictedchrominance value storage means 14 are predicted values resulting fromthe prediction process concerning chrominance.

Reference chrominance value storage means 12 and predicted chrominancevalue storage means 14 are provided respectively for the two types ofchrominances, Cb and Cr.

The total sizes of the reference value storage volumes of referenceluminance value storage means 11 and reference chrominance value storagemeans 12 and the predicted value storage volumes of predicted luminancevalue storage means 13 and predicted chrominance value storage means 14is smaller than the storage volume for the predicted luminance andchrominance values for the entire image size.

Referring to the flowchart in FIG. 7, and also to FIGS. 8-10, theprediction process carried out by prediction means 6 (the predictionprocess of step 14 of FIG. 6) is now described.

In step 20, prediction control means 10 sets a target area 31 forluminance and a target area 51 for chrominance to the target macro blockat the upper left. In step 21, prediction computation means 15 performsprediction computation on the upper left luminance Y1, upper rightluminance Y2, lower left luminance Y3, and lower right luminance Y4 asshown in FIG. 8. The predicted values obtained here are held inpredicted luminance value storage means 13.

In step 22, prediction computation means 15 performs predictioncomputation concerning the chrominance values Cb and Cr as shown in FIG.10. The predicted values obtained here are held in predicted chrominancestorage means 14.

If in step 23, prediction control means 10 judges that the target macroblock is not at the right end, prediction computation means 15 returnsthe predicted values to prediction control means 10.

After the processes of steps 15 through 18, shown in FIG. 6, arecompleted based on the abovementioned predicted values, the target macroblock is advanced at step 19, and the process of step 14 is carried outbased on the results of step 13.

In step 20 shown in FIG. 7, target macro block setting means 3 sets thetarget area 31 to the next target macro block as indicated by the arrowN1 (see FIG. 8). The same process is carried out for target area 51. Thesubsequent processes are the same as those described above.

When in step 23 shown in FIG. 7, prediction control means 10 judges thatthe target macro block is at the right end, step 24 is entered.

In step 24, prediction control means 10 copies the DC components and ACcomponents held in the lower stage part 100 of predicted luminance valuestorage means 13 into the line part 23 as indicated by the arrow N2 ofFIG. 9. Although the details will be described later, the reason forperforming copying in this manner is to use the DC components and ACcomponents copied into line part 23 as reference values for theprediction process of the target macro blocks of the next single line.

In step 25, prediction control means 10 copies the DC components and ACcomponents held in predicted chrominance storage means 14 into the linepart 43 as indicated by the arrow N4 of FIG. 10. The reason forperforming copying in this manner is the same as that for the copyingcarried out in step 24.

In step 26, the predicted luminance value storage means 13 and predictedcolor value storage means 14 are shifted by one. That is, since theprediction process for the target macro blocks of a single line havebeen completed, predicted luminance value storage means 13 and predictedcolor value storage means 14 are used to hold the predicted values ofthe target macro blocks of the next single line.

In step 27, prediction control means 10 performs initial setting ofcorner parts 21 and 41 and left parts 22 and 42 in regard to luminanceand chrominance.

Then after the processes of steps 15 through 18, shown in FIG. 6, arecarried out based on the predicted values of the target macro block atthe right end, the target macro block is renewed in step 19 and theprocess of step 14 is carried out based on the results of step 13.

In step 20 shown in FIG. 7, target macro block setting means 3 sets thetarget area 31 to the next target macro block as indicated by the arrowN3 (see FIG. 9). The same is carried out for target area 51. Thesubsequent processes are the same as those described above.

The details of the prediction computation carried out in step 21 shownin FIG. 7 is now described with reference to FIG. 11. In FIG. 11, theportions that are the same as those in FIG. 8 are identified by the samereference designators.

The DC component 101 of luminance Y1 is predicted based on the DCcomponent held in corner part 21, the DC component 501, and the DCcomponent 401. The AC component 201 of luminance Y1 is predicted basedon AC component 502. The AC component 301 of luminance Y1 is predictedbased on AC component 402.

That is, the prediction computation is performed in accordance with theprediction computation principles that were described by way of FIGS. 3through 5. DC component 101 corresponds to being the Fx[0][0] (see step6 of FIG. 5) that is obtained using the PFx[0][0] of block X of FIG. 3.The DC component held in corner part 21 corresponds to being theFb[0][0] of block B of FIG. 3. The DC component 401 corresponds to beingthe Fa[0][0] of block A of FIG. 3. The DC component 501 corresponds tobeing the Fc[0][0] of block C of FIG. 3.

Also, AC component 201 corresponds to being the Fx[j][0] (see step 8 ofFIG. 5) that is obtained using the PFx[j][0] of block X of FIG. 4. TheAC component 502 corresponds to being the Fc[j][0] of block C of FIG. 4.AC component 301 corresponds to being the Fx[0][i] (see step 9 of FIG.5) that is obtained using the PFx[0][i] of block X of FIG. 4. The ACcomponent 402 corresponds to being the Fa[0][i] of block A of FIG. 4.

The DC component 102 of luminance Y2 is predicted based on the DCcomponents 503, 501, and 101. The AC component 202 of luminance Y2 ispredicted based on AC component 504 and the AC component 302 ofluminance Y2 is predicted based on the AC component 301. As with theabove described prediction computation, this prediction computation isalso in accordance with the prediction computation principles that weredescribed using FIGS. 3 through 5.

The DC component 103 of luminance Y3 is predicted based on the DCcomponents 101, 401, and 403. The AC component 203 of luminance Y3 ispredicted based on AC component 201 and the AC component 303 ofluminance Y3 is predicted based on the AC component 404. As with theabove described prediction computation, this prediction computation isalso in accordance with the prediction computation principles that weredescribed using FIGS. 3 through 5.

The DC component 104 of luminance Y4 is predicted based on the DCcomponents 102, 101, and 103. The AC component 204 of luminance Y4 ispredicted based on AC component 202 and the AC component 304 ofluminance Y4 is predicted based on the AC component 303. As with theabove described prediction computation, this prediction computation isalso in accordance with the prediction computation principles that weredescribed using FIGS. 3 through 5.

Likewise, the prediction process is carried out for the other targetblocks in accordance with the prediction computation principles thatwere described using FIGS. 3 through 5. The same applies to theprediction process concerning chrominance.

The meaning of the copying performed in step 24 shown in FIG. 7 is nowdescribed in detail with reference to FIG. 12.

FIG. 12 shows the condition where the target area 31 is set to thetarget block at the left end of the next single line as indicated by thearrow N3 of FIG. 9. In accordance to the prediction computationprinciples that were described with reference to FIGS. 3 through 5, inpredicting the DC component 111, the DC component 103, the DC componentheld in corner part 21, and the DC component 401 must be used.

Likewise, to predict the AC component 211, the AC component 203 must beused. Likewise, to predict the DC component 112, the DC components 104,103, and 111 must be used. Likewise, to predict the AC component 212,the AC component 204 must be used.

In step 26 shown in FIG. 7, although DC components 103, 104, etc., andAC components 203, 204, etc., (predicted values of the first singleline) are held in predicted luminance value storage means 13 untilpredicted luminance value storage means 13 is shifted by one. Afterpredicted luminance value storage means 13 is shifted as shown in FIG.12, the DC components 111, 112, etc., and AC components 211, 212, etc.,(the predicted values for the current single line) are written over theprevious contents.

Thus if DC components 103 and 104 and AC components 203 and 204 are tobe used to predict the DC components 111 and 112 and AC components 211and 212, the DC components 103 and 104 and AC components 203 and 204must be held in a memory area besides predicted luminance value storagemeans 13.

For this reason, DC components 103 and 104 and AC components 203 and 204are copied into line part 23. In the present case, DC component 104 isoverwritten into the area of line part 23 in which DC component 501 isheld, AC component 203 is overwritten into the area in which ACcomponent 502 is held, DC component 103 is overwritten into the area inwhich DC component 503 is held, and AC component 204 is overwritten intothe area in which AC component 504 is held.

The above is the meaning of the copying process in step 24 shown in FIG.7. The copying process in step 25 also has the same meaning.

As a result of such copying, DC component 111 is predicted using the DCcomponent 103 that has been copied over DC component 501, the DCcomponent held in corner part 21, and the DC component 401 held in leftpart 22.

DC component 112 is predicted using the DC component 104 that has beencopied over DC component 503, the DC component 103 that has been copiedover DC component 501, and the DC component 111.

AC component 211 is predicted using the AC component 203 that has beencopied over AC component 502. AC component 212 is predicted using the ACcomponent 204 that has been copied over AC component 504.

The DC components and AC components, which have been copied from thelower stage part 100 of predicted luminance value storage means 13 intoline part 23 are thus used as reference values in the prediction of thetarget macro blocks subsequent to the target macro blocks of the firstsingle line. The same applies to the prediction process for chrominance.

The DC components and AC components that have been input in line part 23in the initial setting process are used as reference values in theprediction of the target macro blocks of the first single line (see step11 of FIG. 6). The same applies to the prediction process forchrominance.

In accordance with the prediction computation principles described withreference to FIGS. 3 through 5, AC component 311 is predicted using ACcomponent 402, DC component 113 is predicted using DC components 111,401, and 403, and AC component 313 is predicted using AC component 404.

That is, the DC components 401 and 403 and AC components 402 and 404,which are held in left part 22, are used to predict the DC componentsand AC components at the left end (DC components 101, 103, 111, 113,etc., and AC components 301, 303, 311, 313, etc.). Also, as has beenmentioned above, the DC component held in corner part 21 is used in theprediction of the DC components at the upper left of the target macroblock at the left end (DC components 101, 111, etc). The same applies tothe prediction process for chrominance.

Thus with Embodiment 1, the line part 23 of reference luminance valuestorage means 11 consists of areas that hold the DC components and ACcomponents necessary for the prediction process of the target macroblocks of a single line of image size S.

After the prediction process of the target macro blocks of the currentsingle line is completed, the DC components and AC components necessaryfor the prediction process of the target macro blocks of the next singleline are copied into the line part 23 in which the DC components and ACcomponents that have already been used in the prediction process of thetarget macro blocks of the current single line are held.

The DC components and AC components that have thus been copied into linepart 23 are used in the prediction process of the target macro blocks ofthe next single line.

Thus by performing the prediction process while repeating copying in theabove-described manner, the storage volume for the DC components and ACcomponents (reference values) in reference luminance value storage means11 is reduced. In addition, the area required for storing the DCcomponents and AC components (predicted values) for the entire imagesize is not required in predicted luminance value storage means 13. Thesame can be said of the prediction process for chrominance.

As a result, the memory area used in the prediction process in theprocess of decoding a coded image signal is reduced.

Also, for the four luminance components of each target macro block, linepart 23 has an area allocated for two sets of DC components and ACcomponents. Left part 22 is allocated for two sets of DC components andAC components for the four luminance components of the target macroblock.

The case where a target macro block is arranged of four luminancecomponents in accordance with the standard specifications is thusaccommodated adequately. This is especially favorable for the predictionprocess in the decoding of image signals that have been coded inaccordance with the MPEG-4 specifications.

Also, line part 23 of reference luminance value storage means 11consists of areas that hold the DC components and AC componentsnecessary for the prediction process of the target macro blocks of asingle line in image size S. That is, reference luminance value storagemeans 11 consists of areas in which the DC components and AC componentsof the respective positions are held in a process in which the targetmacro block is moved by one line. The same applies to the referencechrominance value storage means 12.

The copying by prediction control means 10 is performed in a batch ofone line at time to thereby simplify the process.

Although five target macro blocks were contained in a single line ofimage size S in the above, the present invention is not limited thereto,and the magnitude of the image size can be set arbitrarily and thenumber of macro blocks contained in a single line may differaccordingly. Even in such cases, this invention provides theabove-described effects regardless of image size.

(Embodiment 2)

The overall arrangement of the image decoding device of Embodiment 2 ofthis invention is the same as that of the image decoding device of FIG.1, and the prediction means of the image decoding device of Embodiment 2is the same as that of the prediction means shown in FIG. 2.

Also, the principles of DC/AC prediction (principles of predictioncomputation) carried out by the prediction means of Embodiment 2 are thesame as the prediction computation principles that were described usingFIGS. 3 through 5. The overall flow of the processes carried out by theimage decoding device of Embodiment 2 are also the same as the flowshown in FIG. 6.

The image decoding device of Embodiment 2 differs from the imagedecoding device of Embodiment 1 in the method of the prediction processcarried out by prediction means 6. A description that mainly concernsthis aspect is now given with reference to FIGS. 13 through 19.

First, the details of reference luminance value storage means 11 andpredicted luminance value storage means 13, which are shown in FIG. 2and are necessary for carrying out the prediction process, aredescribed.

As shown in FIG. 14, reference luminance value storage means 11 has aline part 63, which holds the DC components and AC components of asingle line in image size S, a corner part 61, which holds a single DCcomponent, and a left part 62, which holds DC components and ACcomponents. The DC components and AC components held by referenceluminance value storage means 11 are reference values to be used in theprocess of carrying out the prediction process concerning luminance.

For the four luminance components of each target macro block, line part63 has an area allocated for two sets of DC components and ACcomponents. To be more specific, in the present example, since there arefive target macro blocks in a single line in image size S, line part 63consists of memory areas that hold ten DC components and ten ACcomponents.

Left part 62 is allocated for two sets of DC components and ACcomponents for the four luminance components of the target macro block.To be more specific, left part 62 consists of memory areas that hold twoDC components and two AC components.

Predicted luminance value storage means 13 consists of memory areas thathold the DC components and AC components of the luminance of one targetmacro block. This is the major point of difference with respect to thepredicted luminance value storage means 13 of Embodiment 1.

The DC components and AC components held in predicted luminance valuestorage means 13 are predicted values obtained as a result of theprediction process concerning luminance.

The details of the reference chrominance value storage means 12 andpredicted chrominance value storage means 14, which are shown in FIG. 2and are necessary for carrying out the prediction process, is nowdescribed.

As shown in FIG. 19, reference chrominance value storage means 12 has aline part 83, which holds the DC components and AC components of asingle line in image size S, a corner part 81, which holds a single DCcomponent, and a left part 82, which holds DC components and ACcomponents. The DC components and AC components held by referencechrominance value storage means 12 are reference values to be used inthe process of carrying out the prediction process concerningchrominance.

For each target macro block, line part 83 has an area allocated for oneset of DC components and AC components. To be more specific, in thepresent example, since there are five target macro blocks in a singleline in image size S, line part 83 consists of memory areas that holdfive DC components and five AC components.

Left part 82 is allocated for one set of DC components and AC componentsfor the target macro block. To be more specific, left part 82 consistsof memory areas that hold one DC component and one AC component.

Predicted chrominance value storage means 14 consists of memory areasthat hold the DC components and AC components of the chrominance of onetarget macro block. This is the major point of difference with respectto the predicted chrominance value storage means 14 of Embodiment 1. TheDC components and AC components held in predicted chrominance valuestorage means 14 are predicted values obtained as a result of theprediction process concerning chrominance.

Also, reference chrominance value storage means 12 and predictedchrominance value storage means 14 are provided respectively for the twotypes of chrominance, Cb and Cr.

The total of the reference value storage volumes of reference luminancevalue storage means 11 and reference chrominance value storage means 12and the predicted value storage volumes of predicted luminance valuestorage means 13 and predicted chrominance value storage means 14 issmaller than the storage volume for the predicted luminance andchrominance values for the entire image size.

The prediction process carried out by prediction means 6 (the predictionprocess of step 14 of FIG. 6) is now described by way of FIGS. 13through 19.

As shown in FIGS. 13 and 14, in step 30, prediction computation means 15performs prediction computation on luminance Y1 at the upper left of thetarget macro block. This prediction computation is performed in the samemanner as in Embodiment 1. The predicted values obtained here are heldin predicted luminance value storage means 13.

If in step 31, the target macro block is at the left end, step 32 isentered. In step 32, prediction control means 10 performs initialsetting of the corner part 61 and left part 62, which are shown in FIG.14, and the corner part 81 and left part 82, which are shown in FIG. 19.

In step 34, prediction computation is performed on the upper rightluminance Y2, lower left luminance Y3, and lower right luminance Y4 ofthe target macro block as shown in FIG. 15. This prediction computationis also performed in the same manner as in Embodiment 1. The predictedvalues obtained here are held in predicted luminance value storage means13.

In step 35, prediction control means 10 copies the DC component 103 andAC component 203 for the lower left luminance Y3 from predictedluminance storage means 13 into line part 63 as indicated by the arrowN5.

The reason for performing copying in this manner is the same as that forthe copying into line part 23 (see FIG. 9) carried out with Embodiment1.

That is, the DC component and AC component are copied into line part 63to be used as reference values for the prediction process of the targetmacro blocks of the next single line.

In step 36, prediction control means 10 copies the DC component 102 andAC component 302 of the upper right luminance Y2 and the DC component104 and AC component 304 of the lower right luminance Y4 from predictedluminance value storage means 13 to left part 62 as indicated by thearrow N7 of FIG. 16. Although the meaning of this copying is describedin detail later, the copying is performed to use the DC components 102and 104 and AC components 302 and 304 as reference values in theprediction process of the adjacent target macro block to the right.

In step 37, prediction computation means 15 performs predictioncomputation on the target macro block on each of the chrominance valuesCb and Cr. The predicted values obtained here are held in predictedchrominance value storage means 14.

In step 38, prediction control means 10 copies the DC components and ACcomponents of the chrominance from predicted chrominance value storagemeans 14 into line part 83 as indicated by the arrow N9 of FIG. 19. Themeaning of this copying is the same as that of the copying carried outin step 35.

Prediction control means 10 also copies the DC components and ACcomponents of the chrominance into left part 82 as indicated by thearrow N10 of FIG. 19. The meaning of this copying is the same as that ofthe copying carried out in step 36.

Prediction computation means 15 then returns the predicted valuesobtained in the above to prediction control means 10. After theprocesses of steps 15 through 18 shown in FIG. 6 are then preformedbased on these predicted values, the target macro block is renewed atstep 19 and the process of step 14 is carried out based on the resultsof step 13.

Here, in step 30 of FIG. 13, prediction computation is performedconcerning the luminance Y5 at the upper left of the renewed targetmacro block as shown in FIG. 17. The predicted values obtained here areoverwritten into the area in which the predicted values concerningluminance Y1 are held in predicted luminance value storage means 13.

Since predicted luminance value storage means 13 can only hold thepredicted values for one target macro block, when the prediction processfor one target macro block is completed, it is used to hold thepredicted values (values for the luminance Y5 in the present case) ofthe next target macro block.

In the present case, since the target macro block is not at the left endin step 31, step 33 is entered. In step 33, prediction control means 10copies the DC component 104 and AC component 204 of the lower rightluminance Y4 of the target macro block from predicted luminance valuestorage means 13 to line part 63 as indicated by the arrow N8 of FIG.17.

Here, the DC component 104 and AC component 204 are copied into an areaof line part 63 that is adjacent to the area into which the DC component103 and AC component 203 of luminance Y3 have been copied. The meaningof this copying is the same as that of the copying performed in step 35.

Step 34 is then entered and prediction computation means 15 performs theprediction computation concerning luminance Y6, Y7, and Y8 as shown inFIG. 18.

The obtained predicted values concerning the luminance Y6, Y7, and Y8are respectively overwritten into the areas of predicted luminance valuestorage means 13 in which the luminance Y2, Y3, and Y4 were held.Thereafter, the processes of step 35 through step 38 are performed inthe same manner as described above.

The meaning of the copying performed in step 36 shown in FIG. 13 is nowdescribed in detail using FIG. 20. As shown in FIG. 20, in accordancewith the prediction computation principles that were described usingFIGS. 3 through 5, in predicting the DC component 121, the DC component505, DC component 503, and DC component 102 are necessary.

Likewise, to predict the AC component 321, the AC component 302 must beused. Likewise, to predict the DC component 123, the DC components 121,102, and 104 must be used. Likewise, to predict the AC component 323,the AC component 304 must be used.

However, though prior to performing the prediction process for thetarget macro block (luminance Y5 to Y8), the predicted values of thetarget macro block (luminance Y1 to Y4) indicated by the broken linesare held in predicted luminance value storage means 13, since predictedluminance value storage means 13 can hold only the predicted values forone target macro block, in performing the prediction process for thetarget macro block (luminance Y5 to Y8), the predicted values of thetarget macro block (luminance Y5 to Y8) become written over thepreviously held values.

Thus in the case where the DC components 102 and 104 and AC components302 and 304 of the target macro block (luminance Y1 to Y4) are to beused to predict the DC components 121 and 123 and AC components 321 and323 of the target macro block (luminance Y5 to Y8), the DC components102 and 104 and AC components 302 and 304 must be held in a memory areabesides predicted luminance value storage means 13.

For this reason, the DC components 102 and 104 and AC components 302 and304 of the immediately prior block (luminance Y2 and Y4) that is locatedadjacent to the left side of the target macro block (luminance Y5 to Y8)are copied into left part 62.

In the present case, DC component 102 is overwritten into the area inwhich DC component 401 is held, AC component 302 is overwritten into thearea in which AC component 402 is held, DC component 104 is overwritteninto the area in which DC component 403 is held, and AC component 304 isoverwritten into the area in which AC component 404 is held.

The above indicates the meaning of the copying process in step 36 shownin FIG. 13. The copying into the left part 82, which is carried out instep 38 has the same meaning.

As a result of such copying, DC component 121 is predicted using the DCcomponents 505 and 503 and the DC component 102 that has been copied andis held in the area in which DC component 401 was held.

DC component 123 is predicted using the DC component 121, the DCcomponent 102 that has been copied and is held in the area in which DCcomponent 401 was held, and the DC component 104 that has been copiedand is held in the area in which DC component 403 was held.

AC component 321 is predicted using the AC component 302 that has beencopied and is held in the area in which AC component 402 was held. Also,AC component 323 is predicted using the AC component 304 that has beencopied and is held in the area in which AC component 404 was held.

Thus in the prediction concerning target macro blocks besides the macroblock at the left end, the DC components and AC components that havebeen copied from predicted luminance value storage means 13 into leftpart 62 are used as reference values. The same applies to the predictionprocess for chrominance.

Meanwhile, for the prediction of the target macro block at the left end,the DC components and AC components that were input into left part 62 inthe process of initial setting are used as the reference values (seestep 11 of FIG. 6 and step 32 of FIG. 13). The same applies to theprediction process for chrominance.

Also, as with Embodiment 1, the DC component stored in corner part 61 isused in the prediction of the upper left DC component (DC component 101,etc.) of the target macro block at the left end. The same applies to theprediction process for chrominance.

As shown in FIGS. 15 and 17, the process of copying the DC componentsand AC components of luminance Y3 and Y4 into line part 63 differsgreatly from Embodiment 1, in which the DC components and AC componentsfor a single line are copied in a batch into line part 23 (see FIG. 9).

That is, as shown in FIG. 17, the copying of the DC component 104 and ACcomponent 204 of luminance Y4 is performed after the prediction processconcerning luminance Y5 is completed, and is not performed at the sametime as the copying of DC component 103 and AC component 203 ofluminance Y3, shown in FIG. 15.

The meaning of this is now described with reference to FIG. 20.

When for example, the DC component 121 of luminance Y5 is to bepredicted, the DC component 503, which has been input into line part 63in the process of initial setting in step 11 of FIG. 6, is used.

However, if the DC component 104 of luminance Y4 is copied into the areaof line part 63 in which DC component 503 is held before the predictionof the DC component 121 of luminance Y5, the prediction of the DCcomponent 121 of luminance Y5 will be performed using the DC component104 that has been copied into line part 63.

This would not be in accordance with the prediction computationprinciples described with reference to FIGS. 3 through 5. The DCcomponent 104 and AC component 204 of luminance Y4 are thus copied intoline part 63 after the prediction process concerning luminance Y5.

Thus, as in Embodiment 1, the DC components and AC components necessaryfor the prediction process of the target macro blocks of the next singleline are copied into line part 63 of Embodiment 2 as shown in FIGS. 15and 17.

The DC components and AC components that have been copied into line part63 can thus be used as reference values in the prediction process of thetarget macro blocks of the next single line.

The performing of the prediction process while repeating such copyingthus eliminates the need to hold the DC components and AC componentsnecessary for the prediction process of the target macro blocks of thenext single line in predicted luminance value storage means 13.

With Embodiment 2, when the prediction process of the current singletarget block (luminance Y1 to Y4) is completed, the DC components 102and 104 and AC components 302 and 304, which are the prediction results,in other words, the DC components 102 and 104 and AC components 302 and304, which are necessary for the prediction process of the next targetmacro block (luminance Y5 to Y8) (which is adjacent to the right) arecopied from predicted luminance value storage means 13 to left part 62as shown in FIG. 16.

The DC components 102 and 104 and AC components 302 and 304, which havebeen copied into left part 62, can thus be used in the prediction of thenext target macro block (luminance Y5 to Y8) (which is adjacent to theright).

The performing of the prediction process while repeating such copyingeliminates the need to hold the DC components 102 and 104 and ACcomponents 302 and 304 of the current target block (luminance Y1 to Y4)in predicted luminance value storage means 13 for the prediction processof the next target macro block (luminance Y5 to Y8) (which is adjacentto the right).

As a result of the above, with Embodiment 2, only an area for holdingthe DC components and AC components for a single target block needs tobe secured in predicted luminance value storage means 13. The same canbe said of predicted chrominance storage means 14.

The memory area to be used in the prediction process in the decoding ofa coded image signal can thus be lessened significantly.

For the same reasons given above for Embodiment 1, the case where atarget macro block is arranged of four luminance components inaccordance with the standard specifications can thus be accommodatedadequately. This is especially favorable for the prediction process inthe decoding of image signals that have been coded in accordance withthe MPEG-4 specifications.

Also, although five target macro blocks were contained in a single linein image size S in the above, the present invention is not limitedthereto. The magnitude of the image size can be set arbitrarily and thenumber of macro blocks contained in a single line may differaccordingly. Even in such cases, this invention provides theabove-described effects regardless of image size.

By this invention, the memory used for the prediction process is reducedsignificantly to enable the consumption of power in an image processorto be limited, the duration of continuous use of equipment equipped withthe processor to be extended, and the production cost of the processorto be reduced.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments, and that various changesand modifications may be effected therein by one skilled in the artwithout departing from the scope or spirit of the invention as definedin the appended claims.

1. An image decoding device, which decodes in target macro block units,coded image signals that are coded by DCT, quantization, andvariable-length coding, comprising: a target macro block setting meansfor setting a target macro block to be decoded currently from among anentire image to identify a target macro block; a target macro blockextraction means for performing a variable length decoding process and azigzag scan on said coded image signal and extracts data of said targetmacro block to produce an extracted target macro block data, aprediction means for performing a DC/AC prediction process on saidextracted target macro block data, an inverse quantization means forperforming inverse quantization on said target macro block data thathave been subject to said prediction process, and an inverse DCT meansfor performing inverse DCT on said target macro block data that havebeen subject to inverse quantization and outputs an image of said targetmacro block; said prediction means includes a reference value storagemeans; said reference value storage means including means for holdingreference values that are necessary for said prediction processperformed on said target macro block; a prediction computation means forperforming prediction computation based on reference values of saidreference value storage means; a predicted value storage means forholding results of said prediction computation; a prediction controlmeans for controlling said reference value storage means, saidprediction computation means, and said predicted value storage means;and a total storage volume of said reference value storage means andsaid predicted value storage means is smaller than a storage volume ofpredicted values for an entire image size, and said prediction controlmeans includes means for copying data, necessary for said predictioncomputation of a next target macro block, from said predicted valuestorage means to said reference value storage means.
 2. An imagedecoding device as set forth in claim 1, wherein: said reference valuestorage means includes means for holding a line part; said line partincludes DC components and AC components for a single line in said imagesize; said reference value storage means further includes means forholding a corner part; said corner part including one DC component saidreference value storage means further includes a left part; and saidleft part includes DC components and AC components of a block locatedimmediately prior and adjacently to a left side of said target macroblock.
 3. An image decoding device as set forth in claim 2, wherein forfour luminance components of each target macro block, said line partincludes an area allocated for two sets of DC components and ACcomponents.
 4. An image decoding device as set forth in claim 2, whereinsaid left part is allocated for two sets of DC components and ACcomponents for said four luminance components of said target macroblock.
 5. An image decoding device as set forth in claim 3, wherein saidleft part is allocated for two sets of DC components and AC componentsfor said four luminance components of said target macro block.
 6. Animage decoding device as set forth in claim 1, wherein said predictedvalue storage means includes areas for holding DC components and ACcomponents of respective positions in a process in which said targetmacro block is moved by one line in said image size.
 7. An imagedecoding device as set forth in claim 1, wherein said predicted valuestorage means includes an area for holding said DC components and ACcomponents of one target macro block.
 8. An image decoding device as setforth in claim 2, wherein said predicted value storage means includes anarea for holding said DC components and AC components of one targetmacro block.
 9. An image decoding device as set forth in claim 3,wherein said predicted value storage means includes an area for holdingsaid DC components and AC components of one target macro block.
 10. Animage decoding device as set forth in claim 4, wherein said predictedvalue storage means includes an area for holding said DC components andAC components of one target macro block.
 11. An image decoding methodfor decoding, in target macro block units, of coded image signals thatare coded by DCT, quantization, and variable-length coding, comprising:setting a target macro block to be decoded currently, to identify a settarget block; performing a variable length decoding process and a zigzagscan on said coded image signal and extracting data of said target macroblock to produce extracted target macro block data; performing a DC/ACprediction process on said extracted target macro block data; performinginverse quantization on target macro block data that have been subjectto the prediction process; performing inverse DCT on said target macroblock data that have been subject to inverse quantization and outputtingan image of this target macro block; said step of performing a DC/ACprediction process includes performing said DC/AC prediction processwhile copying data, necessary for said prediction computation of a nexttarget block, into a storage area; and said storage area being smallerthan a storage area that would be required for storage of data of anentire image.